Field of the Invention
The invention relates to a system for accurately measuring the amplitude and relative phase of RF signals. More specifically, the invention relates to such a system which is based on synchronous sampling.
A) Operation of a conventional vector analyzer
Basically, a vector analyzer is a system which is used to measure the complex amplitude (i.e. the amplitude and relative phase) of one or more signals in the frequency domain. It is the basis for instruments such as vector network analyzers, vector voltmeters and modulation analyzers. Conventionally, a vector analyzer uses a heterodyne technique, R. A. Witte and J. W. Daniels, "An advanced 5 Hz to 200 MHz network analyzer", Hewlett Packard Journal, pp. 4-16, November 1984: the signals to be processed, whose frequency .function..sub.IN may be any value inside the working range of the instrument, are first converted to a fixed intermediate frequency .function..sub.IF by mixers. The mixers are non-linear devices with two input ports (IN, LO) and one output port (IF) configured in such a way as to produce an output signal at the frequency .function..sub.IF through the relation EQU .function..sub.IF =.+-.(.function..sub.IN -m.function..sub.LO)(1)
where .function..sub.LO is the frequency of the signal applied at the LO port. m is an integer equal to 1 for fundamental mixing and greater than 1 for "harmonic mixing". Using a bandpass filter at the IF port, the analyzer can be tuned to a frequency .function..sub.IN by applying the appropriate LO frequency such that eq. (1) is satisfied.
The system is arranged to be linear with respect to the input IN, so that the amplitude and relative phase of the input signals are preserved by this mixing process. The resulting IF signals are filtered, amplified and generally frequency converted again, and finally go to a synchronous detector for quadrature and phase demodulation. Sweeping, i.e. tuning the analyzer at a frequency which changes over time, is accomplished by sweeping .function..sub.LO in such a way that .function..sub.IF is constant.
Some of the most important specifications for today's vector analyzers are its drift (or stability) and dynamic linearity. Other parameters which affect the accuracy of the instrument such as load match errors and frequency response errors are effectively cancelled out by normalization, calibration and vector correction techniques implemented in software. In practice the linearity is limited by the IF chain and the synchronous detector, and is generally about 0.02 dB for available commercial instruments. In the case of drift, it is mostly due to the variation in the transfer function of the mixer with temperature and aging, and typical values are 0.01 to 0.05 dB.
Our analyzer uses synchronous sampling rather than harmonic mixing to make the frequency conversion to a fixed IF frequency. Using this technique, we show that it is possible to improve the dynamic linearity and stability, at the expense of other factors which are not critical for many applications, such as measurement speed and spurious signal rejection.
B) Sampling techniques
Sampling systems were introduced for the observation of high speed repetitive signals, N. S. Nahman, "The Measurement of Baseband Pulse Rise Times of Less than 10.sup.-9 Second" Proceedings of the IEEE, Vol. 55, No. 6, June 1967, pp. 855-864. In these systems, a sampling gate, usually made of high speed Schottky diodes, is used to take a quasi-instantaneous snapshot of the input voltage at the time it receives a "sampling strobe". By taking a series of such samples over time it is possible to reconstruct the input waveform, provided it is repetitive and some known time relationship exists between the sampling strobe and the signal. The main interest of these techniques is that only the sampling gate determines the equivalent bandwidth of the system. The rest of the circuitry only has to process low frequency signals, contrary to a real time instrument. Some sampling systems now have over 30 GHz equivalent time bandwidth and around 1 psec time resolution.
Depending on the specific time relationship required by the instrument between the signal to acquire and the sampling strobe, we distinguish between three types of sampling techniques:
Sequential sampling: the signal to be measured goes to a trigger unit in addition to being applied to the sampling gate. When the system is ready to take a sample, it will wait until a trigger event occurs. The sampling strobe will be sent a given delay later by the sampling system; in order to get the complete waveform the delay is increased slightly for each sample. This technique is often used for TDR (Time Domain Reflectomerry) systems. PA1 Random sampling: the sampling strobe is issued at a constant rate .function..sub.S independent of the signal characteristics. When a trigger event occurs the time between it and the next sampling strobe is measured accurately and this value is used to compute the time index for preceding and succeeding samples. When a sufficiently high number of trigger events have occurred the time indexes will be nearly evenly distributed over the complete range from 0 to 1/.function..sub.S, in which case the waveform can be displayed with sufficient resolution. Many modern digital oscilloscopes use random sampling to achieve a "repetitive bandwidth" greater than their real time sampling rate. PA1 Synchronous sampling: is defined as a technique wherein the sampling strobe is applied at a constant .function..sub.S and the input signal has a repetition frequency .function..sub.IN which possess a known mathematical relationship with .function..sub.S. It is not necessary to be concerned about triggering, as a known synchronism exists between each sampling strobe and the input signal. Although not explicitly mentioned, it is used in special applications such as those found in N. D. Faulkner and E. V. Mestre, "Subharmonic sampling for the measurement of short-term stability of microwave oscillators", IEEE Trans. Instr. Meas., Vol. IM-32, pp. 208-213, March 1983 and P. A. Weisskopf, "Subharmonic sampling of microwave signal processing requirements", Microwave Journal, pp. 239-247, May 1992. PA1 a sampling system comprising a plurality of sampling gates, each sampling gate having an input terminal, an output terminal and a control terminal; PA1 a sampling strobe synthesizer having an output terminal connected to the control terminals of said sampling gates; PA1 a discrete time signal processor (DTSP) having a plurality of input terminals, respective ones of the output terminals of said sampling gates being connected to respective ones of the input terminals of said DTSP, said DTSP also including a like plurality of channels, each channel being associated with a respective input terminal of said DTSP, and a plurality of output terminals; PA1 a reference clock; PA1 wherein:
Synchronous sampling has many resemblances to harmonic mixing: even the circuits of a harmonic mixer and a sampling gate may share some common points. The differences that exist are: 1) the excitation of a sampling gate is generally at a much lower frequency than that of a harmonic mixer (.function..sub.S &lt;.function..sub.LO) and, more important 2) the output signal of a harmonic mixer is a continuous time signal whereas the output of a sampling gate is a sequence of samples.
Although B. Gestblom, "The sampling oscilloscope in dielectric frequency domain spectroscopy", J. Phys. E: Sci. Instrum., Col. 15, pp. 87-90, 1982 and R. H. Cole, "Bridge sampling methods for admittance measurements from 500 KHz to 5 GHz", IEEE Trans. Instr. Meas., Vol IM-32, pp. 42-47, March 1983, have discussed the use of sequential sampling oscilloscopes for complex amplitude measurement in the frequency domain for simple systems, the present invention provides much more functionality in terms of automation, accuracy and effectiveness.